Different techniques are being developed to circumvent the problem of sealing around the pipe and wireline cable for example, logging while drilling (LWD), memory shuttles conveyed by pumping them down inside pipe or using slim tools such as RST conveyed with cable inside drill pipe. So far as known to applicant, the problems with conveying a full suite of open hole logging tools in an under-balanced horizontal well remains unsolved.
Key benefits to pursue UBD include a high rate of penetration (ROP) within short radius horizontal drilling programs where up to 3-4 times reduction in drilling times have been reported. Although ROP has been a huge success, the potential of UBD to transform into a reservoir characterization and recovery technology has been masked by the limitations in current state of the art to perform advanced imaging services in an under-balanced well with a rotary blow out preventer (RBOP) stack at the surface necessary to accomplish UBD. This has forced some drilling companies and owners to insist that the well be killed between trips thus jeopardizing the benefits and increased productivity value of performing UBD. This technique will also enhance the impact of wireline logging, especially high-end technologies not available with logging while drilling programs (LWD), in horizontal under-balanced wells. This would permit development of combined formation evaluation and production evaluation programs for wireline services in the under-balanced horizontal drilling market.
The technique may also be useful in other situations where we may have a possibility to log using a jointed pipe in a workover well where either the well pressure is such that we could not convey by pumping the tools down or the trajectory of the well made it impossible for us to convey the tools using just gravity. In many cases, CT logging is considered as the only viable solution; however, this technique could be used with normal jointed pipe as a readily accessible solution not dependent on CT and its availability. This could improve wireline access to horizontal evaluation market.
The standard drilling practice is to maintain an overbalance pressure at all times acting on the formation while drilling which is intentionally kept above the formation pore pressure however below the formation fracture/failure pressure to prevent unwanted influx of fluids into the well bore and to prevent a catastrophic blow out. Due to the overbalance pressure, fluids as well as solids infiltrate and invade into the reservoir rocks where potential hydrocarbons will be produced from. This results in damaging the virgin reservoir rock and thereby reducing the productivity of the well.
Under-balanced drilling is a drilling methodology in which the hydrostatic and dynamic pressures of mud are purposively kept below the formation pore pressure while maintaining complete well control at surface thereby allowing the well to flow or produce while drilling proceeds while preventing risks arising out of handling pressure and hydrocarbons flow at surface. A well drilled under-balanced achieves better productivity as compared to a well drilled over-balanced provided that the under-balance is maintained at all times from drilling till final completion and production. Under-balanced drilling and completion is especially suited in depleted fields where the formation pressure has reduced considerably after years of production and it is either impossible to drill through due to mechanical failure of reservoir rocks or significant damage is caused due to massive invasion. Either problem can justify an investment in under-balanced drilling and completion techniques.
Horizontal wells are a commonly used technique in the industry. Horizontal wells enable a long interval of reservoir to be contacted in a single well thereby improving the productivity and enhancing reservoir recovery economics. Horizontal well logging techniques have also evolved. While drilling measurements allow accurate well placement, in certain types of reservoirs and depending on the evaluation objectives requested by the owner, there still remains a strong need to perform wireline logging operations involving high resolution imaging, magnetic resonance measurements, as well as downhole fluid analysis and sampling services—most of which are currently not available with LWD. For such cases, the industry has developed a method conveniently termed in current literature as “tough logging conditions” (TLC) which enables the tools to be conveyed on drill pipe while also maintaining an electrical connection to the surface unit using a standard wireline cable. The method, in summary, involves conveying tools in the well using drill pipe till just above the last casing shoe. A cable side door entry sub is then inserted in drill string to allow the cable to be rigged up and to enter inside the drill pipe through the side entry sub and further connect to the tools already down hole. The cable is tied up or fixed at the side entry sub and further both cable and drill pipe are simultaneously conveyed down to perform logging operations. A standard feature of a TLC system is that a certain length of cable, equal to the length of the logging interval as a minimum, ends up being outside the pipe located between the rig floor and down to point in the drill string where the cable enters the drill pipe i.e. the side entry sub.
Under-balanced drilling is especially suited for horizontal wells because formation damage in horizontal over-balanced wells can be very significant due to the long contact length and contact time between reservoir rocks and drilling fluids as well as constant scraping of filter cake by the drill pipe lying down on the low side of the horizontal. Therefore, significant productivity is lost due to formation damage in horizontal wells. The industry has therefore realized the need to design technologies that are able to perform under-balanced drilling in horizontal wells to lower the formation damage that would otherwise be caused if the well was drilled over-balanced and thereby achieve higher productivity.
Another advantage of performing UBD is that it allows productivity data to be obtained while the well is being drilled. In wells where N2 injection is made to achieve under-balance, the accuracy of such productivity data is questionable due to the lack of measurements available along the entire length of drill pipe and fluctuations in the fluid density as well as flow contributions and pressures in the entire length of the open hole section. Various techniques such as “testing while under-balanced drilling” have been introduced however they rely on the accuracy of four-phase models to determine reservoir permeability and productivity. The accuracy of such models to determine the four-phase fluid behaviors and flow characteristics in a complex well trajectory is also very limited. Hence, productivity information obtained from under-balanced wells while drilling has a large margin of error; however, any information assists the operators determination, albeit with low accuracy, of the ultimate true potential of a well with no damage to the well.
In vertical wells, once UBD is finished, the well can be logged in an under-balanced state using conventional logging techniques utilizing surface pressure control systems rigged up through the standard rig blow out prevention stack to accurately determine the reservoir productivity. Supply of N2, if required, can be provided by a parasitic string inserted for this specific purpose. However, in horizontal wells, the standard TLC technique as used in over-balanced drilling environment suffers from a serious limitation as a certain cable section must be kept outside of the drill pipe in length equal to the interval being logged, located between rig floor and down hole cable side entry sub which cannot be sealed around as the annular BOPs are not designed to seal around a pipe with a wire outside it and any attempt to do so could damage the cable and jeopardize the whole operation. This means that advanced services logging operations such as high resolution imaging, production logging measurements such as downhole flow rates, phase hold ups and zonal contributions from reservoir and others not available using LWD or memory option cannot be performed with standard surface set up which is a serious disadvantage for the exploration and production (E&P) operator. In some cases coil tubing with electric cable could be an option however the ability of coil tubing to push a heavy suite of open hole logging tools all the way to TD in a long horizontal open hole is a serious short coming, not to mention the added complexity, risk and investment needed to carry out such an operation. Without wireline log data, it is not possible for the E&P operators to accurately determine the ultimate true productivity potential of the well. It is also not possible to optimize completion design based on accurate productivity profiles. It is also not possible to improve the accuracy of while drilling productivity measurements. There is a need for a new system that would give E&P operators easy access to wireline technologies in horizontal under-balanced well construction.
The invention is designed to address all the short comings in current state of art of the logging during the under-balanced well construction process, thereby allowing operators to reap full benefits of investment in under-balanced drilling from increase ROP to enhanced characterization and increase recovery.
The main objective of this invention is therefore to provide apparatus and methods of using the apparatus to perform the equivalent of TLC logging operation in an under-balanced horizontal well in order to achieve the following benefits which are three fold:                1. Perform logging operations in an under-balanced horizontal well without killing it, thereby ensuring any higher well productivity benefits achievable due to reduced or no formation damage are not jeopardized. The well could be either just drilled or an old well that requires logging with rig on site.        2. Direct determination of reservoir characterization parameters such as flow, pressure, PI, sweet spots (permeable micro fractures) early in the process of well construction and with required accuracy in order to determine the Ultimate True Productivity Potential (UTPP) of an undamaged horizontal (or multi-lateral) well drilled using under-balanced drilling technique, thus enabling E&P operators to book additional reserves under a recoverable category.        3. Under-balanced well completion optimization, more specifically in a fractured reservoir, by optimal placement of zonal isolation materials/packers using well productivity profiles obtained early during well construction process as input.        